In injection molding, hot melt is injected into the tool (injection mold) by an injection molding machine either directly or via a hot-channel distribution system by means of a hot-channel nozzle. The tool consists of an injection side, accommodating the hot-channel nozzle, in which the outer mold (cavity) of the part of synthetic material is as a rule located, and an ejection side with the inner mold (core). The region in which the hot-channel nozzle connects to the tool and to the part of synthetic material is called the “gate.” In injection molding, the front, tool-side part of the hot-channel nozzle with the gate region is inserted into a recess of the tool. In the rear region, the nozzle is supported axially on the hot channel distribution block and is fixed in the tool with the front gate part by precision sealing. There, the nozzle is exposed to severe mechanical and thermal stresses. Since the gate region of the nozzle consequently is subject to rapid wear, it is not designed as an integral part of the hot-channel nozzle, but as a replaceable tool insert.
The connection of the tool insert to the nozzle and to the tool may be designed in a variety of ways. The extent of work for the toolmaker is variable, depending upon the accomplishment. In addition, the necessary thermal separation between nozzle and tool plays an important role in these accomplishments. After conclusion of the actual injection process, the melt injected into the tool should solidify rapidly, while the melt retained in the nozzle tip should remain liquid. This means that a temperature difference of about 200° C. must be obtained between the two regions mentioned.
A known embodiment of the tool insert consists in that it is capable of being screwed into the front (tool-side) end of the nozzle. This accomplishment requires the least work by the toolmaker. He need only insert the nozzle bore mass into the tool and provide a fitting bore for the gate region. The disadvantage of this accomplishment consists in that, for one thing, owing to screwing of the tool insert into the nozzle, undesirable good thermal conduction from the nozzle shaft to the front of the tool insert takes place. An additional disadvantage of this accomplishment is that, owing to heating of the hot-channel distribution block into which the nozzle shafts are screwed, and heating of the nozzles, thermal variations in position result, which negatively influence the precise position of the nozzles and the tool inserts screwed into them in regard to the tool. A third disadvantage is that the front contact surface between the tool insert and the cavity of the tool is not axially symmetrical but, for example, is designed beveled. In this case, the nozzle must be introduced into the fitting bore of the tool with the tool insert in a precise angular position about its longitudinal axis. With regard to the required tight screwed joint between tool insert and nozzle, on the one hand, and nozzle shaft and hot-channel distribution block on the other, this is in practice not achievable after renewed assembly and disassembly has taken place with such screwing actions.
A second known accomplishment consists, instead of a separate tool insert, of working the gate region directly into the tool. Here, positioning and thermal separation can be obtained in good fashion. However, the disadvantage of this accomplishment is the high cost of fabrication for the toolmaker. He must work hemispherical surfaces, conical surfaces, shoulders and a variety of fits into the tool. An additional disadvantage of this accomplishment consists in that, upon wear of the gate, the entire injection-side of the tool or individual cavities must be replaced or repaired.
In a third known accomplishment, the extent of work for the toolmaker is reduced as compared with the said second accomplishment, in that the gate is produced by an insert in the tool. In this accomplishment, thermal separation is good, and a specified angular position can be obtained by a device securing against rotation. In case of wear, the tool insert can simply be replaced by a fresh tool insert. But the work for the toolmaker in this accomplishment is also greater than in the accomplishment first mentioned.